1. Field of the Invention
The present invention relates to an apparatus for extruding a plasticized polymer material, particularly an extruder with a reservoir reducing screw tip.
2. Related Art
In conventional extruders, extrudate is transported with a screw conveyor through a barrel portion of the extruder and out a dispensing outlet. The extrudate may be used, for example, to produce chips, flakes or pellets of solid polymer for subsequent molding. Alternately, the extrudate may be extruded directly into a mold. The area between the barrel portion and the dispensing outlet is referred to herein as an “adapter” or “reservoir.”
The flow area of the extrudate between the barrel portion of the screw conveyor and the dispensing outlet in conventional extruders is large as compared with the flow area in the barrel or through the dispensing outlet. As a result, there is a large volume of deadspace formed in the adapter, typically immediately before the dispensing outlet. As the extrudate proceeds from the barrel portion into the adapter, expansion occurs which results in the changing of melt flow patterns of the extrudate. The forces from the extruder applied to moving the extrudate are lessened and therefore the motion of the extrudate slows. As a result, areas of the polymer extrudate stagnates against the adapter and layers of previously extruded polymer build-up in the adapter. Consequently, the opening of the adapter becomes smaller and the amount of extrudate being output decreases.
In addition, the stagnation of the extrudate increases the amount of time the extrudate spends in the region of the dispensing outlet. As a result, the pressure necessary to force the extrudate through the adapter increases and causes the temperature of the extruded polymer to increase. As the stagnant portion or layers of built-up polymer cool, additional heat must be continually supplied to the extrudate to improve the laminar polymer flow as the stagnant portion cools. Thus, higher temperatures are applied to the extruded polymer and the extruder must apply a higher force to move the extrudate. These factors further provide a back pressing throughout the extruder and contribute to the increased back pressure on the screw conveyor, particularly on the flights of the screw conveyor, and consequently, increase the failure rate of the screw conveyor.
In some extruders, a screw tip is attached to the end of the screw conveyor and extends into a die to that forms the extruded structure, for example, in an extruded pipe. Such a known extruder is described in U.S. Pat. No. 3,870,451 to Gokcen where, as illustrated in FIG. 1 thereof, a die 18 with a conically shaped chamber 24 receives a mandrel 29 with a generally conical upstream portion that has the same diameter of the conveyor screw 15 at its widest point. Surrounding the die are a plurality of heating bands 36. The mandrel taught by Gokcen forms a cylindrical pipe that is extruded into a sizing structure with cooling fluid to remove heat. Thus, Gokcen teaches a mandrel that acts as a die pin forming a parison with the conical chamber of the die, and is not concerned with increasing the output of the extruded material or reducing heat applied to the extruded material.
In some injection molding apparatuses, a screw tip is attached to the end of the screw conveyor and moves in and out of the nozzle body to assist the flow of plastic material from a low pressure environment through the nozzle opening into a mold. The screw tip of injection molding apparatuses also provides a seal at the nozzle body to maintain the high pressure necessary for injection molding. Thus, screw tips employed with known injection molding apparatuses are designed to stop the flow from the low pressure environment to the high pressure environment. While these injection molding screw tips may reduce the flow area of the extrudate, their design does not increase the amount of extrudate output.